Removal of Chromium Ions by Fe3O4‐containing Polypyrrole Matrix and Environmental Applications

In this work, the PPy/Fe₃O₄@TiO₂ composite was synthesized and characterized by X‐ray diffraction (XRD), scanning electron microscopy, transmission electron microscopy, and magnetic measurements (using a vibrating sample magnetometer). The adsorption performance of PPy/Fe₃O₄@TiO₂ composite for Cr(VI) ions was evaluated by UV irradiation. The effects of pH, adsorbent dose, contact time, and the initial concentration on the adsorption performance of Cr(VI) onto PPy/Fe₃O₄@TiO₂ were investigated. The maximum adsorption capacity of Cr(VI) upon doped PPy/Fe₃O₄@TiO₂ is 85.30 mg/g at room temperature. The total adsorption process likely follows the Langmuir model and pseudo‐second‐order kinetics. Our study suggests that the PPy/Fe₃O₄@TiO₂ composite can be efficiently used for the adsorption of Cr(VI) ions.     

Highly efficient adsorption of Cr(VI) from aqueous solution by Fe3+ impregnated biochar

Biochar (BC) has been widely used as a low-cost adsorbent for the removal of contaminants from water and soil. However, the adsorption ability of BC towards heavy metal oxyanions (e.g., Cr(VI)) is relatively low due to the negatively charged surface of BC. In this study, pristine BC was impregnated with Fe³⁺ to improve its Cr(VI) adsorption capability. Fe³⁺-impregnated BC (Fe³⁺-BC) was successfully synthesized by a simple impregnation method and used for the removal of Cr(VI) from aqueous solution. Various factors affecting the adsorption, such as impregnation ratio, pH, adsorbent dosage, contact time, temperature, and the presence of humic acid, were investigated in detail. Results showed that Fe³⁺-BC had strong adsorption ability to Cr(VI) with a maximum adsorption capacity of 197.80 mg/g, which were not only significantly higher than that of the pristine BC, but also were superior to many previously reported adsorbents. It was favorable for Cr(VI) adsorption under the condition of acidic and high temperature. The adsorption data obeyed Sips and Langmuir isotherms and the kinetic data were well described by the pseudo-first-order kinetic model. The results herein revealed that the Fe³⁺-impregnated BC had a good potential as a highly efficient material for adsorption of Cr(VI) from aqueous solution.     

Preparation of carbon coated Fe3O4 nanoparticles for magnetic separation of uranium

Uranium(VI) was removed from aqueous solutions using carbon coated Fe₃O₄ nanoparticles (Fe₃O₄@C). Batch experiments were conducted to study the effects of initial pH, shaking time and temperature on uranium sorption efficiency. It was found that the maximum adsorption capacity of the Fe₃O₄@C toward uranium(VI) was ∼120.20 mg g⁻¹ when the initial uranium(VI) concentration was 100 mg L⁻¹, displaying a high efficiency for the removal of uranium(VI) ions. Kinetics of the uranium(VI) removal is found to follow pseudo-second-order rate equation. In addition, the uranium(VI)-loaded Fe₃O₄@C nanoparticles can be recovered easily from aqueous solution by magnetic separation and regenerated by acid treatment. Present study suggested that magnetic Fe₃O₄@C composite particles can be used as an effective and recyclable adsorbent for the removal of uranium(VI) from aqueous solutions.     

Sorption of uranium on magnetite nanoparticles

Magnetite (Fe₃O₄) nanoparticle was synthesized using a solid state mechanochemical method and used for studying the sorption of uranium(VI) from aqueous solution onto the nanomaterial. The synthesized product is characterized using SEM, XRD and XPS. The particles were found to be largely agglomerated. XPS analysis showed that Fe(II)/Fe(III) ratio of the product is 0.58. Sorption of uranium on the synthesized nanomaterials was studied as a function of various operational parameters such as pH, initial metal ion concentration, ionic strength and contact time. pH studies showed that uranium sorption on magnetite is maximum in neutral solution. Uranium sorption onto magnetite showed two step kinetics, an initial fast sorption completing in 4–6 h followed by a slow uptake extending to several days. XPS analysis of the nanoparticle after sorption of uranium showed presence of the reduced species U(IV) on the nanoparticle surface. Fe(II)/Fe(III) ratio of the nanoparticle after uranium sorption was found to be 0.48, lower than the initial value indicating that some of the ferrous ion might be oxidized in the presence of uranium(VI). Uranium sorption studies were also conducted with effluent from ammonium diuranate precipitation process having a uranium concentration of about 4 ppm. 42% removal was observed during 6 h of equilibration.     

Removal of trace Cr(VI) from water using chitosan-iron nanowires in porous anodic alumina

Chitosan-iron nanowires in porous anodic alumina (PAA) have been successfully prepared under ambient conditions as an adsorbent. The adsorbent was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy and N₂-BET surface area. The results showed that PAA can disperse and protect Fe⁰ nanorods from oxidation. The adsorption characteristics of trace Cr(VI) onto adsorbent have been examined at different initial Cr(VI) concentrations with pH 5. Batch adsorption studies show that the removal percentage of adsorbent for the removal of trace Cr(VI) is strongly dependent on the initial Cr(VI) concentrations. Langmuir and Freundlich isotherm models were used to analyze the experiment data. The adsorption of trace Cr(VI) by adsorbent is well modeled by the Langmuir isotherm and the maximum adsorption capacity of Cr(VI) is calculated as 123.95 mg/g which is very closed to the experiment results. Intraparticle diffusion study shows that the intraparticle diffusion of adsorbent is not the sole rate-controlling step. The negative value of Gibbs free energy change, ΔG^o, indicated that the process of Cr(VI) onto adsorbent was spontaneous. This work has demonstrated that chitosan-iron nanowires in porous anodic alumina as an adsorbent has promising potential for heavy metal removal at trace level.     

Synthesis of Calcium Silicate Hydrate from Coal Gangue for Cr(VI) and Cu(II) Removal from Aqueous Solution

Both the accumulation of coal gangue and potentially toxic elements in aqueous solution have caused biological damage to the surrounding ecosystem of the Huainan coal mining field. In this study, coal gangue was used to synthesize calcium silicate hydrate (C-S-H) to remove Cr(VI) and Cu(II)from aqueous solutions and aqueous solution. The optimum parameters for C-S-H synthesis were 700 °C for 1 h and a Ca/Si molar ratio of 1.0. Quantitative sorption analysis was done at variable temperature, C-S-H dosages, solution pH, initial concentrations of metals, and reaction time. The solution pH was precisely controlled by a pH meter. The adsorption temperature was controlled by a thermostatic gas bath oscillator. The error of solution temperature was controlled at ± 0.3, compared with the adsorption temperature. For Cr(VI) and Cu(II), the optimum initial concentration, temperature, and reaction time were 200 mg/L, 40 °C and 90 min, pH 2 and 0.1 g C-S-H for Cr(VI), pH 6 and 0.07 g C-S-H for Cu(II), respectively. The maximum adsorption capacities of Cr(VI) and Cu(II) were 68.03 and 70.42 mg·g⁻¹, respectively. Furthermore, the concentrations of Cu(II) and Cr(VI) in aqueous solution could meet the surface water quality standards in China. The adsorption mechanism of Cu(II) and Cr(VI) onto C-S-H were reduction, electrostatic interaction, chelation interaction, and surface complexation. It was found that C-S-H is an environmentally friendly adsorbent for effective removal of metals from aqueous solution through different mechanisms.     

Sorption of U(VI) onto a decarbonated calcareous soil

Sorption of U(VI) from aqueous solution to decarbonated calcareous soil (DCS) was studied under ambient conditions using batch technique. Soil samples were characterized by XRD, FT-IR and SEM in detail and the effects of pH, solid-to-liquid ratio (m/V), temperature, contact time, fulvic acid (FA), CO₂ and carbonates on U(VI) sorption to calcareous soil were also studied in detail using batch technique. The results from experimental techniques showed that sorption of U(VI) on DCS was significantly influenced by pH values of the aqueous phase, indicating a formation of inner-sphere complexes at solid–liquid interface, and increased with increasing temperature, suggesting the sorption process was endothermic and spontaneous. Compared to Freundlich model, sorption of U(VI) to DCS was simulated better with Langmuir model. The sorption equilibrium could be quickly achieved within 5 h, and sorption results fitted pseudo-second-order model well. The presence of FA in sorption system enhanced U(VI) sorption at low pH and reduced U(VI) sorption at high pH values. In absence of FA, the sorption of U(VI) onto DCS was an irreversible process, while the presence of FA reinforced the U(VI) desorption process reversible. The presence of CO₂ decreased U(VI) sorption largely at pH >8, which might due to a weakly adsorbable formation of Ca₂UO₂(CO₃)₃ complex in aqueous phase.     

The MOF+ Technique: A Significant Synergic Effect Enables High Performance Chromate Removal

A significant synergic effect between a metal–organic framework (MOF) and Fe₂SO₄, the so‐called MOF⁺ technique, is exploited for the first time to remove toxic chromate from aqueous solutions. The results show that relative to the pristine MOF samples (no detectable chromate removal), the MOF⁺ method enables super performance, giving a 796 Cr mg g⁻¹ adsorption capacity. The value is almost eight‐fold higher than the best value of established MOF adsorbents, and the highest value of all reported porous adsorbents for such use. The adsorption mechanism, unlike the anion‐exchange process that dominates chromate removal in all other MOF adsorbents, as unveiled by X‐ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), is due to the surface formation of Fe_0.75Cr_0.25(OH)₃ nanospheres on the MOF samples.     

Synthesis, characterization, thermodynamic and kinetic investigations on uranium (VI) adsorption using organic-inorganic composites: Zirconyl-molybdopyrophosphate-tributyl phosphate

Zirconyl-molybdopyrophosphate-tributyl phosphate (ZMPP-TBP) was a novel organic-inorganic composite adsorbent prepared by co-precipitation method and used in the adsorption of uranium from aqueous solution in batch adsorption experiments. The as-obtained product was characterized using SEM, energy dispersive X-ray spectroscopy (EDX), XRD and BET-N₂ adsorption measurements. The study had been conducted to investigate the effects of solution pH, temperature, contact time, initial concentration and coexisting ions. A maximum removal of 99.31% was observed for an initial concentration 5 mg/L, at pH 6.0 and an adsorbent dose of 1.0 g/L. The isothermal data were fitted with both Langmuir and Freundlich equations, but the data fitted the former better than the latter. According to the evaluation using the Langmuir equation, the maximum adsorption capacity of uranium (VI) was 196.08 mg/g at 293 K and pH 6.0. The pseudo-first-order kinetic model and pseudo-second-order kinetic model were used to describe the kinetic data, and the pseudo-second-order kinetic model was better. The thermodynamic parameter ΔG was calculated, the negative ΔG values of uranium (VI) at different temperature showed that the adsorption process was spontaneous. The good reusability of ZMPP-TBP also indicated that the ZMPP-TBP was a very promising adsorbent for uranium adsorption from aqueous solution.     

Separation and spectrophotometric determination of very low levels of Cr(VI) in water samples by novel pyridine-functionalized mesoporous silica

A modified SBA-15 mesoporous silica was developed, as an adsorbent, for the removal of Cr(VI) ions from natural-water samples. The effects of experimental parameters, including pH of solution, sample and eluent flow rate, the eluent composition, the eluent volume, and the effect of coexisting ions on the separation and determination of Cr(VI), were investigated. It was shown that Cr(VI) was selectively adsorbed from aqueous solution at pH 3, but Cr(III) could be adsorbed from solution at alkaline pH range. The retained Cr(VI) was eluted with 0.5?mol?L^?1 KCl solution in 0.1?mol?L^?1 Na₂CO₃ subsequently. Under the optimum conditions, the modified mesoporous silica (py-SBA-15) with a high pore diameter exhibited an adsorption capacity of 136?mg?g^?1 and a lower limit of detection than 2.3?µg?L^?1 by using diphenylcarbazide as a chromophorous reagent for the determination of Cr(VI) ions. A preconcentration factor as high as 200 was calculated for Cr(VI). The loaded py-SBA-15 can be reactivated with recovery of more than 98.5% over at least eight cycles. The relative standard deviation (RSD) for Cr(VI) ion recovery was less than 1.8%. Validation of the outlined method was performed by analysing a certified reference material (BCR 544). The proposed method was applied to determine Cr(VI) value in natural and waste water samples successfully.     

Preparation and sorption performance of magnetic 18-crown-6/Fe3O4 nanocomposite for uranium(VI) in solution

A new magnetic nanocomposite material, magnetic 18-crown-6/Fe₃O₄ nanocomposite (MCFN), was prepared for the removal of U(VI) from aqueous solution. The MCFN was composed of Fe₃O₄ nanoparticales modified by covalent attachment of 18-crown-6, which can help the material to be removed easily from solution by magnetic force. As a new adsorbent for U(VI) removal, MCFN was characterized by infrared radiation, scanning electron microscopy with energy dispersive X-ray spectroscopy, vibrating sample magnetometer and thermal gravimetric analysis. Those factors affecting the sorption behavior of U(VI), such as acidity, temperature, initial concentration of U(VI) and the amount of crown ethers were studied by orthogonal experiments. A maximum U(VI) sorption capacity of 91.12 mg g^?1 was achieved at 45 °C, pH 5.5 for 30 min. The experimental results showed that MCFN had great sorption capacity, high selectivity and strong potentiality of enrichment and recovery for U(VI). In summary, MCFN is a promising candidate for U(VI) separation in future practical applications.     

Preparation of calix[4]arene-based sporopollenin and examination of its dichromate sorption ability

The present study describes the preparation of a new calix[4]arene-based sporopollenin material and its application for the removal of Na₂Cr₂O₇ from aqueous solution. The novel calix[4]arene-based sporopollenin material was prepared via the immobilization of dihydrazine amide derivative of p-tert-butylcalix[4]arene (3) onto the modified sporopollenin. The newly prepared calix[4]arene-based sporopollenin is characterized by using different analytical techniques such as FT-IR spectroscopy, scanning electron microscope and Elemental analysis. The batch wise sorption study was carried out to optimize various experimental parameters such as the effect of sorbent dosage, pH, temperature and Cr(VI) anion concentration. It has been found that the sorption of Cr(VI) anion on calix[4]arene-based sporopollenin was highly pH dependent and maximum sorption was achieved at pH 1.5. The sorption behavior was also evaluated by Langmuir, Freundlich and Dubinin Radushkevich isotherms. The value of correlation coefficient (R ²) showed a good agreement with Freundlich isotherm model. Result of study demonstrated that calix[4]arene-based sporopollenin proved to be highly effective for the removal of Cr(VI).     

Simultaneous Determinations of Fe2+ and Fe3+ in Ferrous Sulfate Destined for Pharmaceutical Preparations

ABSTRACT

Only the simultaneous analysis of the amount of Fe^2? and the possible presence of Fe³⁺ in ferrous sulfate heptahydrated (FeSO_4. 7 H₂O) can guarantee covered tablets, drops or syrups with insured quality. This work suggests that these analysis are accomplished through spectrophotometric method by use of the 1, 10-phenanthroline as chelate. Thus, at 510 nm the absorption is only due to the complex of the ligand with Fe^2? and at 390 nm the absorption is indicative of Fe^2+/3+ coordinated. Pharmaceutical raw matter and covered tablets were analyzed. Techniques of thermogravimetric analysis (TG-DTG) were used for the knowledge of the regions of loss of water of the FeSO₄. 7 H₂O for the relationship to its stoichiometry.     

Sustainable Chromium (VI) Removal from Contaminated Groundwater Using Nano-Magnetite-Modified Biochar via Rapid Microwave Synthesis

This study developed a nano-magnetite-modified biochar material (m-biochar) using a simple and rapid in situ synthesis method via microwave treatment, and systematically investigated the removal capability and mechanism of chromium (VI) by this m-biochar from contaminated groundwater. The m-biochar was fabricated from reed residues and magnetically modified by nano-Fe₃O₄. The results from scanning electron microscopy (SEM) and X-ray diffraction (XRD) characterisations confirmed the successful doping of nano-Fe₃O₄ on the biochar with an improved porous structure. The synthesised m-biochar exhibited significantly higher maximum adsorption capacity of 9.92 mg/g compared with that (8.03 mg/g) of the pristine biochar. The adsorption kinetics followed the pseudo-second-order model and the intraparticle diffusion model, which indicated that the overall adsorption rate of Cr(VI) was governed by the processes of chemical adsorption, liquid film diffusion and intramolecular diffusion. The increasing of the pH from 3 to 11 significantly affected the Cr(VI) adsorption, where the capabilities decreased from 9.92 mg/g to 0.435 mg/g and 8.03 mg/g to 0.095 mg/g for the m-biochar and pristine biochar, respectively. Moreover, the adsorption mechanisms of Cr(VI) by m-biochar were evaluated and confirmed to include the pathways of electrostatic adsorption, reduction and complexation. This study highlighted an effective synthesis method to prepare a superior Cr(VI) adsorbent, which could contribute to the effective remediation of heavy metal contaminations in the groundwater.     

Adsorption of Cr(VI) using activated neem leaves: kinetic studies

In the present study, adsorbent is prepared from neem leaves and used for Cr(VI) removal from aqueous solutions. Neem leaves are activated by giving heat treatment and with the use of concentrated hydrochloric acid (36.5 wt%). The activated neem leaves are further treated with 100 mmol of copper solution. Batch adsorption studies demonstrate that the adsorbent prepared from neem leaves has a significant capacity for adsorption of Cr(VI) from aqueous solution. The parameters investigated in this study include pH, contact time, initial Cr(VI) concentration and adsorbent dosage. The adsorption of Cr(VI) is found to be maximum (99%) at low values of pH in the range of 1-3. A small amount of the neem leaves adsorbent (10 g/l) could remove as much as 99% of Cr(VI) from a solution of initial concentration 50 mg/l. The adsorption process of Cr(VI) is tested with Langmuir isotherm model. Application of the Langmuir isotherm to the system yielded maximum adsorption capacity of 62.97 mg/g. The dimensionless equilibrium parameter, R _L, signifies a favorable adsorption of Cr(VI) on neem leaves adsorbent and is found to be between 0.0155 and 0.888 (0<R _L<1). The adsorption process follows second order kinetics and the corresponding rate constant is found to be 0.00137 g/(mg) (min).     

Chemical Oxidative Degradation of Acridine Orange Dye in Aqueous Solution by Fenton's Reagent

Degradation of acridine orange (AO) in aqueous solution by Fenton's reagent (Fe²⁺ and H₂O₂) was investigated. The effects of different reaction parameters such as initial AO concentration, pH value of solution, ferrous concentration, hydrogen peroxide concentration, and the presence of chloride ion on the oxidative degradation of AO were investigated. Under optimum conditions, 2 mM H₂O₂, 0.4 mM Fe²⁺ and pH 3.0, the initial 0.2 mM AO solution was reduced by 95.8% within 10 min. The primary intermediates of the degradation reaction of AO were identified. The analytical results indicated that the N‐de‐methylation degradation of AO dye took place in a stepwise manner to yield mono‐, di‐, tri‐, and tetra‐N‐de‐methylated AO species generated during the Fenton process. The probable degradation pathways were proposed and discussed.     

A facile fabrication of MOF for selective removal of chromium (III) from aqueous solution

A simple method for fabricating a metal organic framework (MOF: HKUST-1) as sorbent for selective removal of chromium (III) from aqueous solution is discussed. The structure and morphology of HKUST-1 was identified by fourier transform infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), the powder X-ray diffraction (XRD) and N₂ adsorption-desorption (BET) analysis. Its removal process of chromium (III) and chromium (VI) on HKUST-1 was assessed systematically under various conditions such as pH value, shaking time and initial concentration of chromium (III). At pH 6.0–8.0, HKUST-1 were selective towards chromium (III) but hardly chromium (VI). Kinetic parameters fitted well with pseudo-second-order model and adsorption progress was described by Langmuir isotherm equations and spontaneous and endothermic according to the results of thermodynamics studies (?G?<?0, ?H?>?0, ?S?>?0).     

Application of polyaniline/bacterial extracellular polysaccharide nanocomposite for removal and detoxification of Cr(VI)

Polyaniline/bacterial extracellular polysaccharide (Pn/EPS) nanocomposite was prepared by in situ polymerization of aniline using ammonium peroxydisulfate as oxidant. Transmission electron micrograph showed that the surface of the nanocomposite was rough, providing good possibility for adsorption of Cr(VI). Under optimized conditions, the nanocomposite removed 97.3 % (25 mg L^?1) of Cr(VI) from aqueous solution. The Freundlich isotherm model and pseudo-first order rate expression better described the adsorption equilibrium of Pn/EPS nanocomposite. X-ray diffractogram peak for Cr₂O₃ (2θ = 24.5) in the nanocomposite confirmed the reduction of Cr(VI). Fourier transform infrared spectroscopy pattern of the nanocomposite confirmed the ionic interaction between Cr species and surface functional groups. The results of the study indicate that Pn/EPS nanocomposite could be used for the removal and detoxification of Cr(VI) from aqueous solution.     

Removal of chromium(VI) from wastewater using Sorel's cement

Summary Synthetic Sorel's cement [3Mg(OH)₂ ^. MgCl₂ ^. 8H₂O], is used as a new adsorbent material for removal of chromium(VI) ion from wastewater effluents. Parameters including contact time, adsorbent dosage and pH are examined and optimized. The equilibrium data are fitted very well to the Langmuir and Freundlich isotherms rather than linear. The adsorption isotherm indicates that the monolayer coverage is 21.4 mg Cr(VI) ion per g of Sorel's cement. The adsorbent is considered as a better replacement technology for removal of Cr(VI) ion from aqueous solutions due to its low cost, good efficiency, fast kinetics, and simple preparation. It offers remarkable efficiency for Cr(VI) removal from wastewater compared with many other natural and synthetic adsorbents.     

Synthesis of Mg–Al–Fe–NO3 layered double hydroxides via a mechano-hydrothermal route

Mg–Al–Fe–NO₃ layered double hydroxides (LDHs) with a constant Mg²⁺/(Al³⁺ + Fe³⁺) molar ratio but varying Al³⁺/Fe³⁺ molar ratios were successfully synthesized by a mechano-hydrothermal (MHT) method from Mg(OH)₂, Al(OH)₃ and Fe(NO₃)₃·9H₂O or Mg(NO₃)₂·6H₂O as starting materials. The resulting LDHs (MHT-LDHs) were characterized by XRD, TEM, SEM, FT-IR, and zeta potential, size distribution and specific surface area analyses. It was found that pre-milling played a key role in the LDH formation during subsequent hydrothermal treatment. The MHT route is advantageous in terms of low reaction temperature compared with the conventional hydrothermal method, and the target products are of high crystallinity and good dispersion compared with the conventional mechanochemical (MC) method. The MHT-LDHs had higher specific surface area and zeta potential, and lower hydrodynamic diameter than LDHs obtained by MC method (MC-LDHs). Furthermore, the removal of Cr(VI) from aqueous solutions using the LDHs was examined, showing that the MHT-LDHs are of higher removal efficiency than MC-LDHs for the heavy metal pollutant.